The image quality of images formed by digital image forming devices such as printers and copiers has been greatly improved in recent years and it is possible to use these devices to readily print high-quality images. The reduction in the cost of high-performance scanners, printers and copiers and image processing by computer have made it possible for anyone to obtain desired printed matter with facility. One consequence is the illegal copying of printed matter such as documents, images and photographs. In order to prevent or inhibit the unauthorized use of printed matter by such illegal copying, therefore, access control information in the form of watermark information is embedded in the printed matter.
The access control function generally is implemented by embedding access control information in printed matter in such a manner that it is not visible to the eye, by embedding a bitmap pattern (glyph code, DD code, etc.), which corresponds to the access control information, in the margin of a document, or by scrambling the document image using code. Common methods of implementing the embedding of access control information in such a manner that it will be invisible to the eye include embedding the access control information by controlling the amount of space in an alphabetic character string; rotating characters and embedding the access control information in conformity with the amount of rotation; and enlarging or reducing characters and embedding the access control information in conformity with the enlargement or reduction rate.
FIG. 21 is a diagram useful in describing an example in which access control information is embedded by controlling the amount of space between words in an alphabetic character string. Such a space is indicated at 1701 in FIG. 21. The space is made p←(1+p)(p+s)/s, s←(1−p)(p+s)/2 if a watermark bit to be embedded is “0” and is made p←(1−p) (p+s)/2, s←(1+p)(p+s) if a watermark bit to be embedded is “1”.
FIGS. 22 and 23 are diagrams illustrating an example in which a character is rotated and access control information is embedded in conformity with the amount of rotation. FIG. 22 illustrates a character before it is rotated and FIG. 22 illustrates a character before it is rotated and FIG. 23 the character after it is rotated. The angle θ through which the character is rotated is indicated at 1901 in FIG. 23.
FIG. 24 is a diagram illustrating an example in which access control information is embedded by enlarging or reducing a character. Numerals 2001 and 2002 denote the original character width and the character width after reduction, respectively. The access control information is embedded in conformity with such enlargement or reduction.
With these methods of embedding access control information, however, the original character or image is clearly deformed and the result is degradation of the original character or image.
Further, with these methods of embedding access control information, it is necessary to read the printed matter with high precision and to read the amount of space between characters, the angle of rotation of a character or the size of a character in accurate fashion in order to detect the access control information that has been embedded. If printing is performed with a small character size and at a high resolution, therefore, it is very difficult to detect the access control information that has been embedded in printed matter.